Automatic direction finder



Nov. 1, 1960 R. coLvlN ETAL AUTOMATIC DIRECTION FINDER Filed Dec. 3.1958 JOEL L. EKSTRW INVENTOR; RUEL CONAN BY y ' y MANE "259585867AU'EQMAHC PECTON FINDER Rnel Colvin, Washington, @.C., and Joel L.Ekstrom, Baltimore, :Md., Aassignors i to lThe :Bendix Corporation,aicorpora'tion offDelaware rEtiled-.De- 3, A1958. Sel." 9- .27.715883tclams. 1(761. 643-117) This` invention, relates *generally .to lantonviatic direction Vfindingtradio,receivers iand morefparticularly.to receivers of the type.employing ,la sense antenna .and ,a rotatableloop .antenna c which tis Vautornatically c. oriented `to indicate thedirection fromtthe:location oftheradio receiver to the source of radiowavessbeing received.

.Various .automatic onelofthe standard navigational aidsemployed inpresent day aircraft. .The .F.gener-al acceptance .of `equipment of thistypeis; due` in part to ,the I.excellentperformance which `hast beenobtained ,withqthe present Fday commercially availableautomatic'direction #finders ,which Vwhen employedrwith `chartsindicating thelocations ,of the Atransmitting stations on-theradio rangepermitapilot to ldetermine the f-bearingto a .seleqned `statio'nffor anycparticular Vlocationwhere .he,maybe flying. vAsthe.volume ofair traiiicincreases'andI fthe" number of additionalptypes fof flight servicearegnstituted, --thegneedforl ter reliability in the navigationalnaids as fwell as.yforredlelcedlsine1 weight and decreasad ueost,df'ftheequipment employed vbecome more demanding. `ill/bile .the priorart `direction ,is Well lknown-that dete ion Tof signals sufficient toobtain avbearing indicationis.adverselyaieted by conditions ofVinterference .suchas caused .byeflyingin the neighborhoodoffrnanymade-interference caused by radiat- .ing instrumentalities,suchas .radar'orthe lille ror natural sources of interference-such asprecipitation static ,and the radiofinterference normallyaccompanyinga.thunderstorm. ln both *of these tsituationsthe needfor thereliability of :the navigational aidis ea whereas in `priorartfequ'ipment .the noperational `performance-.of the equipment underthese conditions is sometimes e--low and often unusable lfor reducingthe complexity Vof thevequipmentand improving the `reliability V.0f the:equipment yand e 1the ability of the equipmentfto operate reliablyunder allfcond'itions.

These. and pther. .biestsof. the .intention Wilbe apparent fromthefolllovvingldetailed descriptiontaken in t conjunction with,theuaccompanying drawing wherein-z" ,ltioniinder(QDF).yradiolreeiver,in accordance with the invention; and

n n ection tfind-ing equipments areV Well known. Equipments of thistypetwhichtemploy a `sense fantenna and a rotatable ,loop ,antennaconstitute Fig. 2 is a schematic circuit diagram of a portion of thesystem.

The Vpresent invention employs coherent exalted carrier demodulationwith a `phase lock automatic frequency control system to rprovidedetection of directional radio signals and obtaining a bearingindication. An automatic dlrection finder equipment of this typeoperates reliably in .substantially higher noise levelsY thancommercially available equipment now employed for this purpose.

Referring to the drawing, a system is shown as comprising a conventionalomnidirectional sense antenna V11 and a directional loop antenna 12 ,forthepurpose of sensing respectivelyin an omnidirectional signalV patternanda figure eightsignal pattern. The combination of patterns of thistype .produces the wellfknown effective cardioid pattern for directionfinding by rotating the lloop antenna k12 about avertical axisto apointcorresponding tothe null bearing direction of the cardioid pattern.The signals from the antennas 11 and 12 are coupled to tuned circuitarrangements at the inputsuof broadband amplifiers 13 and 14,respectively. These amplifiers are essentially the same and maycomprise, VVfor example, an input tuned circuityvhich is tuned overftheADF band of ZOO-4Q() lic. `by means y.tof a variable capacitor tuningcircuit in a well-known manner.v .The succeeding stages of theamplifiers At3* and 14 may comprise resistance vcapacitance coupledamplifiers for further amplifying the signal lwhich `is kselectivelylamplified by Vthe first stage yof each channel. Theband width of thelRC coupled amplifiers may be conveniently from a relatively lowfreqnencyv -to Yapproximately 500 lsc. -to pass through the amplifierchannels 13, 14, the signals and the modulation side band componentscontainedtherein. The gang tuning of the input circuitsrfo'r theamplifiers 13,"14 is effected with suiiicient accuracyto maintain phaserelation between the signals which are coupled thereto from the senseand loop antennas, respectively.

An oscillator 15 is provided and is tunable by any conventional meansover the same band over which the amplifiers 13, 14 are tuned.Oscillator 15 is also accurately tracked with the tuning of theamplifiers 13, 14.

VThe output of oscillator 15 .is applied to a 90...phase shifter `16,which may be a capacitively plate vloaded pentode, the output of whichis applied to a phase detector 17 and a phase detector 18. vThe outputof phase detector 18 is `co'upledto a low pass filter 19 the output ofwhich is coupled to reactance tube 21 which in turn is connectedtocontrol the frequency and phase of oscillator 15.

The output signal from the amplifier 13 is applied to an audio frequencymixer amplifier 22 which also has applied thereto via line 23 a signalfrom the oscillator 1S which is not passed through the phase shifter 16.The AF mixer 22 may employ a pentagrid converter tube with theoscillator signal coupled tothe rst grid and the sense channel signalcoupled to the third grid for mixing action therebetween in a mannerknown in the art. The

.mixer amplifier 22may also include a variable resistance element in thecathode circuit of the pentagrid converter tube for adjusting therectification point to obtain no audio signal output when a modulatedsignal is applied to the #3 grid and the oscillator disconnected from.grid #1. The outputv of mixer 22 is applied to a low pass filter 24, anaudio amplifier 25 and a speaker 26 for the 4aural detectionandfidentification of identifying signals and audio communicationrwiththe received station.

The signal from loop channel .amplier ,14 .is preferably, for example,applied to a 400 c.`p.s. chopper 27 whichk is driven synchronouslywithza400 c.p.s. reference voltage .suchasordinarily V'available on aircraft.The output of vthe chopper 27. is applied tothe phase detector 17 ytoproduce bearing information signals when the local oscil- Ilator .15 islocked-to an incoming signal as Awilllbehereinafter described. Theoutput of phase detector 17 consists of a 400 c.p.s. error voltagewhich, after amplification and filtering in an error amplifier 28, hasthe proper phase to drive the loop antenna 12 so as to reduce the loopchannel output to a unilateral null. This servo action is accomplishedby applying the output of the error amplifier 28 to a suitable servomotor 29 which also is connected to the 400 c.p.s. reference whichdrives the chopper 27. The mechanical connection between the motor 29and the loop 12 indicated at 31.

The operation of the system of the present invention is based on theprinciples of coherent exalted carrier demodulation wherein the desiredsignal consisting of a carrier, with or without sidebands, is multipliedin a phase detector by a large locally generated carrier of the samefrequency which is phase locked to the carrier of the incoming signal.This operation may be characterized in terms of the system presentlydisclosed as producing a low frequency output of the detector 17consisting of a D.C. term plus modulation spectra, if present, frequencytranslated directly into the audio spectrum. For example, consider anincoming signal of the following form:

EIN-E1 sin wet (l-l-k sin wmf) Where the local oscillator has afrequency we but may have phase angle relative to the carrier of theinput signal which also has a frequency we. The local oscillator signaltherefore will be:

Eosc=E2 sin (wcti-) The output of a phase detector subject to these twoinput signals is obtained after multiplying and expanding as:

E0 2E2 [cos @+10 eos 0 sin wmt+supersonic terms] in which the D.C. termsis:

E=DCE1 E21; cos 8 and the modulation term is:

EIEE cos 0k 2 EMOD- S111 wmt For best results in such a system the valueof E2 should be quite large with respect to E1 and the phase lock shouldbe good to reduce 0 to zero.

The response of a phase detector as above analyzed (which can beconsidered as a mixer with D.C. IF) in the presence of noise andinterference is such that the chopped D.C. output signal applied to theerror amplifier 28 produces a usuable null signal to the motor 29 in thepresence of relatively high level interference. In the presence ofrandom noise a weak signal is detected in the phase detector 17 with theoutput signal-to-noise ratio improved by 3 db plus an improvement overthe conventional type of diode second detector due to the fact that adiode detector operating in the nonlinear rectification region for smallsignals produces smaller output than the linear frequency translationproperties of the phase detector circuit. For a large signal-to-noiseratio the phase detector is better by 3 db and thus for all conditionsof signal-to-noise ratio the phase detector circuit 17 will givesuperior results because of the avoidance of possible rectificationeffects which may increase the interference power within the audiospectrum.

The operation of the automatic frequency control loop in connection withoscillator 15 to provide a phase lock is accomplished by shifting theoscillator output by 90 in phase shifter 16 such that the oscillator 1Sis maintained in phase with the signals received by the sense antenna11. This result is obtained due to the action of phase detector 18 whichproduces zero output when the two input signals thereto are 90 out ofphase. Since the oscillator 15 has its output shifted by 90 in the phaseshifter 16 the phase detector 18 produces a signal for the feedback loopconsisting of the filter 19 and reactance tube 21 which will maintainthe oscillator 15 at the frequency of signals received on the antenna 11and in phase therewith. The oscillator 15, being thus maintained inphase with the received signal on antenna 11, provides the properreference after passing through phase shifter 16 which has the correctphase to operate in the phase detector 17 to produce the error controlsignals for the amplifier 28.

The phase detectors employed in the present invention may bedemodulators of the type shown in Fig. 2 wherein a triode phase inverter32 applies phase opposed signals from input terminal 37 to diodes 33, 34while a second signal is coupled from terminal 38 to a second pair ofdiodes 35, 36 as shown. In the present invention the shifted oscillatoroutput from the phase shifter 16 is applied to input 37 of the phasedetector 32 while the chopped loop amplifier signal is applied to thediodes 35, 36 via terminal 38l for the second input signal. The audiosignal output is obtained from terminal 39. In the corresponding phasedetector 18 in the AFC circuit, the signal of sense antenna 11 isapplied to the diodes 35, 36 from the sense amplifier 13.

In a system of the type disclosed cross-coupling and leakage between thevarious circuits is required to be reduced to a minimum and for thispurpose adequate shielding and decoupling techniques are to be providedin accordance with good design practice. The output coupling from thesense amplifier 13, the loop amplifier 14 and from the oscillator 15 maybe taken by means of cathode follower amplifiers for the purpose offurther reducing cross-coupling between the various circuits.

With an automatic direction finding equipment constructed in accordancewith the principles of the present invention consistently better resultswere obtained over comparable commercial ADF equipments which employ alocal oscillator and an IF amplifier for amplifying and detecting thereceived signals. The improved results comprise an ability to obtain anaccurate direction indication in the presence of noise where the samesignal was such as to render the indication of the conventional ADFequipment completely unreliable. In addition to the improved operationthe simplicity of the circuits of the present invention permit thereduction of cost, size and weight of what is essentially a portable andoften an airborne system to be reduced to an absolute minimum.Conventional miniaturization and transistorization techniques may bereadily adapted to the present system.

Many other modifications will be apparent to those skilled in the art inthe light of the present teaching and are to be understood as within thespirit and scope of the present invention as defined inthe appendedclaims.

We claim:

1. An automatic direction finding radio receiver comprising a senseantenna, a rotatable loop antenna, a first broad band amplifier coupledto said loop antenna, a second broad band amplifier coupled to saidsense antenna, a variable tuned circuit in the input of each of saidamplifiers for selectively tuning over a predetermined frequency band, avariable frequency oscillator tunable over said predetermined band,means for gang tuning to the same frequency said amplifiers and saidoscillator, first and second phase detectors, means for coupling signalsfrom said oscillator to both said phase detectors. means for couplingthe outputs of said first and second amplifiers to said first and secondphase detectors respectively, means responsive to output signals fromsaid first phase detector for automatically orienting said loop antennato a null position, and an automatic frequency control circuit for saidoscillator responsive to output signals from said second phase detector.

2. An automatic direction finding radio receiver comprising anomnidirectional antenna, a directional antenna having a rotatabledirectional characteristic, a first broad band amplifier coupled to saiddirectional antenna, a second broad band amplifier coupled to saidomnidirec tional antenna, a variable tuned circuit in each of saidamplifiers for selectively tuning over a predetermined frequency band, avariable frequency oscillator tunable over said predetermined band,means for gang tuning to the same frequency said amplifiers and saidoscillator, first and second phase detectors, means for coupling signalsfrom said oscillator to both said phase detectors, means for couplingthe outputs of said rst and second amplifiers to said first and secondphase detectors respectively, means responsive to output signals fromsaid first phase detector for automatically orienting said directionalcharacteristic of said directional antenna to a null position relativeto the bearing of a signal source to which said amplifiers are tuned,and an automatic frequency control circuit for said oscillatorresponsive to output signals from said second phase detector.

3. An automatic direction finding radio receiver comprising anomnidirectional antenna, a directional antenna having an orientabledirectional characteristic, a first broad band amplifier coupled to saiddirectional antenna, a second broad band amplifier coupled to saidomnidirectional antenna, a variable tuned circuit in each of saidamplifiers for selectively tuning over a predetermined frequency baud, avariable frequency oscillator tunable over said predetermined band,means for selectively tuning said amplifiers and said oscillator to thesame frequency corresponding to a radiated signal in said frequencyband, a phase detector, means for coupling oscillator signals andsignals from said first amplffier to said phase detector with a phaserelation for producing a Servo signal of polarity and magnituderepresentative of the sense and magnitude of the angular deviation ofsaid directional characteristic from a bearing direction to the sourceof said radiated signal, means responsive to said servo signal fororienting said directional characteristic to produce a null, and meansresponsive to the output of said second amplifier for controlling saidosclllator to maintain a phase lock condition between the signalgenerated by said oscillator and said radiated signal.

4. Apparatus according to claim 3 in which the signal in said firstamplifier is amplitude modu-lated at a low reference frequency and saidorlenting means includes a motor energized by said reference frequency.

5. An automatic direction finding radio receiver comprising anomnidirectional antenna, a directional antenna having an orientabledirectional characteristic, a first broad band amplifier coupled to saiddirectional antenna, a second broad band amplifier coupled to saidomnidirectional antenna, a variable tuned circuit in each of saidamplifiers for selectively tuning over a predetermined frequency band, avariable frequency oscillator tunable over said predetermined band,means for selectively tuning said amplifiers and said oscillator to thesame frequency corresponding to a radiated signal in said frequencyband, a phase detector, means for coupling oscillator signals andsignals from said first amplifier to said phase detector with a phaserelation for producing a servo signal of polarity and magnituderepresentative of the sense and magnitude of the angular deviation ofsaid directional characteristic from a bearing direction to the sourceof said radiated signal, means responsive to said servo signal fororienting said directional characteristic to produce a null, a secondphase detector, means for coupling phase shi-fted oscillator signals andsignals from said second amplifier to said second phase detector forproducing an output automatic frequency control signal, a low passfilter, a signal controlled frequency determining element coupled tosaid oscillator, circuit means coupling said automatic frequency controlsignal through said low pass filter to said frequency determiningelement to maintain a phase lock condition between the signal generatedby said oscillator and said radiated signal.

6. Apparatus according to claim 5 having in addition an audio frequencymixer, circuit means for coupling signals from the output of said secondamplifier and signals from said oscillator to said audio mixer to detectaudio modulation signals carried by said second amplifier signal, and alow pass audio channel coupled to the output of said audio mixer.

7. An automatic direction finding radio receiver comprising a senseantenna, a rotatable loop antenna, a first broad band amplifier coupledto said loop antenna, a second broad band amplifier coupled to saidsense antenna, a variable tuned amplifier stage in each of saidamplifiers for selectively tuning over a predetermined frequency band, avariable frequency oscillator tunable over said frequency band, meansfor gang tuning said stages and said oscillator to the same frequencycorresponding to a radiated signal in said frequency band, first andsecond phase detectors each having two input circuits and an outputcircuit and being adapted to produce zero output signal for -a ninetydegree phase difference between signals applied to said two inputcircuits and a signal varying in polarity and magnitude in accordancewith deviations from said ninety degree phase difference, means for onehundred percent amplitude modulating signals in said rst amplifier at areference frequency, a ninety degree phase shifter coupled to saidoscillator, means for coupling said amplitude modulated signals andoscillator sgnals from `said phase shifter to the two inputs of saidfirst phase detector, a reference frequency amplifier coupled to theoutput of ysaid first phase detector, a servo motor energized by saidreference frequency, means for rotatably driving said loop antenna withsaid servo motor, means coupling the output of said reference frequencyamplifier to said servo motor to drive said loop to a null for saidradiated s'gnal, means for coupling signals from said second amplifierand oscillator signals from said phase shifter to the two inputs of saidsecond phase detector, a low pass filter, a signal controlled frequencydetermining element coupled to said oscillator, and circuit means forcoupling the output of said second phase detector to said low passfilter and the output of said filter to said frequency determiningelement to maintain a phase lock condition between the signal generatedby said oscillator and said radiated signal.

No references cited.

